One way to start a tactical assessment is by just considering what each of your pieces can do. This includes an automatic glance not only at the diagonals where your bishops sit but also at their ranks and files to see if they contain allied pieces that might be able to deliver discovered attacks. Here Black's bishop on d7 hides the rook on d8 with nothing otherwise blocking the rook’s path to White’s queen. The setup is perfect for a discovery—not quite a discovered check, but an attack based on the same model where the unmasked piece creates a distraction that the enemy must address while the unmasking piece is given two moves to inflict harm. So Black looks for violent moves to make with his bishop and thus finds BxN. Since White will be confronted with a threat to his queen, he won’t be able to recapture with e4xB; Black therefore will be able to withdraw his bishop to safety on his next move and will have gained a piece.